Lactic acid is a specialty chemical used in the food and chemical industries. It is a potential commodity chemical because it can be used to make polymers or hydrogenated to make propylene glycol and other three carbon chemical intermediates. Currently, lactic acid is expensive to make by fermentation because the fermentation produces a crude lactate salt from which the cells and protein have to be removed. To be an economical source of lactic acid, the fermentation has to employ a microorganism which produces high concentrations of lactate from inexpensive substrates. In addition, the recovery, separation and recycle of viable cells and nutrients to the fermentor is required because high cell density and fermentation productivity and low nutrient consumption are important.
Electrodialysis (ED) is a well known separation process where ionized compounds are separated from non-ionized or weakly ionized compounds in aqueous solutions based on transport through ion exchange membranes in an electric field. The process has been used in a commercial scale in the chlor-alkali, desalination, metal-processing, waste-water treatment, pharmaceutical and food processing industries. Since in a fermentation broth the lactate salt is ionized, whereas the carbohydrates and proteins and amino acids are either non-ionized or weakly ionized, recovery and purification of lactate salts from a fermentation broth by electrodialysis is feasible
Recently several papers have been published on the recovery and the purification of lactate from fermentation broths. In the published processes, the broth was either thoroughly cleaned to remove cells and proteins prior to electrodialysis or when the whole broth, containing cells, was used the membrane fouled and led to a loss of efficiency.
Prigent (1) disclosed a method for production of lactate from fermenting whey where the broth was filtered by ultrafiltration to remove the cells and then electrodialyzed. Hongo, Nomura and Iwahara (2) used a whole broth of Lactobacillus delbrueckii IFO 3534 in their electrodialysis apparatus, but discovered that the efficiency was poor due to membrane fouling. These workers (3) then devised a complicated fermentation process where the cells were immobilized in alginate beads and the cell-free broth was fed to the electrodialyzer. The described processes are not practical for commercial use because bacterial cells are very small and removing the cells by high speed centrifugation and/or ultrafiltration is expensive. Cell immobilization in alginate beads also is very expensive and not readily adaptable to commercial use.
For a fermentation process to be economically attractive to produce lactic acid as a commodity chemical, it must be simple and inexpensive. Ideally, the microorganism should grow on low cost substrates and nutrients; the rate of fermentation should be high (high productivity); the lactate concentration in the fermentation broth should be high; and, an inexpensive recovery and purification process should be available.